This invention relates to a motor and an electric pump.
One related motor disclosed, for example, in JP-A-2005-168098, is known. Namely, as shown in FIG. 6, the motor 91 has a cylindrical stator 93 fixed to an inner peripheral surface of a motor case 92. The stator 93 is formed by superimposing together a plurality of electromagnetic steel sheets each made of a steel material which is a magnetic material having high magnetic permeability. As shown also in FIG. 7, a plurality of teeth 93a are formed on and project from an inner peripheral surface of the stator 93 at predetermined intervals, and conductor wires are wound on these teeth to form coils 94.
As shown in FIG. 6, an output shaft 95 is inserted in and rotatably supported on the motor case 92 through a pair of bearings 96a and 96b, and a rotor 97 is mounted on the output shaft 95 for rotation therewith. A tubular magnet 98 is fixedly mounted on an outer peripheral surface of the rotor 97, and this magnet 98 is disposed in such a manner that its outer peripheral surface is opposed to the inner peripheral surface (more accurately, distal end surfaces of the plurality of teeth 93a) of the stator 93 through a slight clearance. Namely, the rotor 97 and the magnet 98 are rotatably supported in a space formed within the stator 93 (more accurately, generally defined by the distal end surfaces of the plurality of teeth 93a). The rotor 97 is rotated by a rotating magnetic field produced by energizing the coils 94.
The motor of this construction has been used as a power source of an electric pump suited for use as a fuel pump, a water pump, an oil pump, a hydraulic pump of a transmission, etc., for a vehicle. For example, an electric pump is disclosed in JP-A-2005-337025, in which in order to reduce the number of component parts and also to achieve a compact design, a single common shaft serves as an output shaft of a motor and also as a drive shaft of a pump.
However, in the above related motor, the stator is mounted on the motor case by press-fitting the former into the latter. Therefore, there has been a fear that following problem might be encountered. Namely, as shown in FIG. 7, the stator 93 is formed such that a transverse cross-section of its outer periphery has a circular shape, and the motor case 92 is formed such that a transverse cross-section of its inner periphery has a circular shape. The outer diameter of the stator 93 is substantially equal to or slightly smaller than the inner diameter of the motor case 92. When the stator 93 is forced or press-fitted into the motor case 92 through one open end thereof with a predetermined force, the stator 93 is fixed to the motor case 92, with its outer peripheral surface held in intimate contact with the inner peripheral surface of the motor case 92.
When the stator 93 is thus press-fitted into the motor case 92 to be fixed thereto, the stator 93 receives a compressive force (external stresses) from the motor case 92 over the entire periphery thereof. As a result, compressive stresses (internal stresses) directed outward from its center occurs in the stator 93. It is commonly known that when such compressive stresses occurs in the stator 93, magnetic properties of the stator 93 are adversely affected, so that iron loss (core loss) increases. In order to suitably prevent the rotation of the stator 93 relative to the motor case 92, it is necessary to secure a certain degree of press-fit interference which is the dimensional difference between the outer diameter of the stator 93 and the inner diameter of the motor case 92. However, as the press-fit interference increases, the compressive stresses occurring in the stator 93 increases and hence the iron loss increase. Further, as the iron loss of the stator 93 increases, the amount of heat generated in the motor 91 increases, which leads to a lowered efficiency of the motor 91. These problems are encountered commonly with the type of motors in which a stator is press-fitted in a motor case to be fixed thereto as in the above related motor.
In recent years, there is still a tendency for vehicles to be computerized, and therefore it has been required to further reduce power consumption. Motors also are not an exception, and it has been required to secure a higher efficiency. Furthermore, in the case where the motor is used as the drive source of the above electric pump, the lowered efficiency of the motor leads to a lowered output power of the electric pump. In this respect, also, it has been strongly desired to secure the still higher efficiency of the motor.